The primary function of hemoglobin is to transport oxygen in vertebrates. Hemoglobin in adult vertebrates has the tetrameric structure `2- 2, and fetal hemoglobin has the structure ` 2- 2. There is considerable interest in the regulation of the expression of fetal hemoglobin, as reactivation of the globin gene can ameliorate the clinical consequences of sickle cell anemia and thalassemia. The embryonic timing of globin expression varies between species. Differences in the control regions of the two globin genes in higher primates, associated with different developmental timing, may shed light on the regulation of the fetal switch. To better understand the regulation of globin expression, the hemoglobin polypeptides of fetal, newborn and adult individuals of the New World monkey, Cebus apella, were investigated. We have used both ESI- and MALDI-MS, coupled with peptide mass mapping, as an analytical tool for analyzing these globin chains. This approach has allowed us to identify the predominant form expressed in fetal hemoglobin. In addition, we have identified and characterized a posttranslationally modified beta chain, a glutathione adduct.